Thesis submitted for Degree of Ph. D. Degree of Ph. D. conferred - 20th October, 192e. UILiEE1lA`1'IUYdS on the GEULOGY of LEV`JIS. Lewis is the largest area in the chain of islands which constitute the Outer Hebrides, some- -times termed the Long Island, and which extend from Barra Head to the Butt of Lewis, a distance of 130 miles. The long tortuous shore of Lewis is mainly a series of high precipitous cliffs penetrated by sea lochs, or breaks, where stretches of sand or shingle and boulder beaches add to the natural attractions of the island. Many islets begird the shores and afford sanctuary to birds and seals. The surface of mid and north Lewis, for over 300 square miles, is an ex- -tensive stretch of bog, moorland, and loch, leaving a cultivated fringe bordering the sea, inhabited by crofting communities. A prospect from the central ridge of the northern moor does not disclose a rock vestige for many miles, and even the streams flow between deep banks of peat, barely disclosing the glacial till. Hilly country in Lewis is found in the/ (2). the high plateau ground of the Park district to the south -east, a central tract from the Oleisham koun- -tain northwards to Roineval, west of iialallan, and the Uig Hills to the south -west; while a few groups and isolated hills rise from surrounding bogs. The old crystalline Lewisian gneiss, of which the island is principally composed, shows a primary north -west strike, but upon this has been superimposed a later terrestrial crust movement which produced a north -north -east strike with dips to the south -east. This latter foliation is predominant from east to west of Lewis. The main Flinty Crush belt, so prominent a feature in the Loñg Island, is chiefly developed in a line trending north -north -east. Obscure foliation planes are observed on high levels of the Park hill -tops and the South Uist Hills, which show north -west strikes. Local movements are evi- -deuced on the north -west shores of Lewis, the struc- tural lines changing to north-west and north -east or east and west with acute folding. Faulting is more prevalent in the crush belts, especially on the east side of the Eye Peninsula and the north -east coast of Lewis./ (a). Lewis. The conglomerate*of Lewis is much faulted within the synclinal trough in which it lies. While the Long Island may be described as essentially a gneissic ridge, there occur outstanding rock types which have a direct bearing on the physical aspect of Lewis. These are: - I. The Granitic Igneous Rocks and Pegmatites. H. The Basic Igneous Rocks of lviaaruig and associa- ted types. III. Series of Graphitic and Siliceous Schist types. IV. The Flinty Crush Belts and related Metamorphic Rocks. a Hebridean Main Belt. b minor Belts. V. Lewis Conglomerates. j. The Granitic Rocks and Pegmatites. mealashhal (1885 feet), the highest peak of the Uig hills, consists of red microcline granite, extensions of which occur to Suainabhal on the west, Thamanabhaidh on the south, and Islivig shore on the west; and similar types are found at Sheabost, 17 miles north -east. In Valtos Glen the course of the granite is shown by a solid belt one mile wide, ex- -posed in precipitous cliffs 200 feet high. Another/ (4). Another Lewisian granite is that of llalbeg (see O. S. Map, 1892), which is a more basic type. Granite too of a grey variety occurs at Bragar Hills, and graphic granite is frequently seen, while the rarer graphite- granite is noted at Loch Almaistean. Several petrographic varieties accompany the granitic intrusions. Pegmatite is very pronounced in veins in all parts of the island, with interesting mineralogical contents. On the east side of Lewis acidic intrusive types such as granite and quartz -pegmatite ramify through the complex of the crush belt areas and in- volve the Lewisian gneiss and granitic gneiss, schists, and paragneisses in greater disorder. (5). Analyses of Granite and other acidic types. A. B. C. B. Si02 72.55 70.68 63.16 65.80 A1203 14.80 15.95 18.49 21.20 Fe2O3 1.27 1.24 2.48 ( tr. Ee0 1.15 0.69 0.99 MgO tr. 0.40 1.08 0.22 (;a0 0.88 2.09 1.84 0.38 Na20 0.68 2.38 7.28 2.09 K2O 8.42 6.06 0.10 10.83 1120 0.95 0.48 3.88 - TiO2 0.11 0.09 - - Mn0 0.08 - 0.29 - 002 - - 0.44 100.89 100.06 100.03 100.52 A. Granite - laaalasbhal Peak, highest hill in Lewis. (Potash microcline felspar. B. Coarse granite - west of Ben lore, Lewis. 0. Felsitic type - belt of 12 feet, Loch Shell, Lewis. D. Coarse pegmatite - Borth Rona island, Lewis. (6). II. The Basic Igneous hocks of Maaruig and Associated hypes. The Maaruig basic mass, including an adjac- -ont islet in Loch Seaforth, is exposed at Maaruig for over one -quarter mile in diameter. One limb from this great igneous centre is traced in a north- west direction by Ardvourlie, Scaladale, Loch Langavat, the north -east corner of Scalabhal, through Loch Mhorsgail, into the hills. In an exposure at Valtos Bay we may have the emergence on the Atlantic side of the iviaaruig mass, a distance of over 20 miles. The idaaruig rock is generally a fresh holo -crystalline olivine gabbro, though in lesser part microcrystalline, and exhibiting derivative forms. There are loose gabbro blacks in the peat, to the south -west of the Maaruig mass, and probably related to it. Other ex- -posures of gabbro types are noted in Lewis. One occurs near Breascleit as a chain of low eminences running north -west for half a mile. It is a greenish holocrystalline rock of uniform texture. Serpentines occur on Crionaig Hill; at Fidigarry; Beinn a Bhuna; and on the west coast: many/ (7). many show lines of asbestos. Hornblendic types occur widely in massive and schistose forms. Garnetiferous hornblende rock, as at Kintaravey, is a common type, and is largely found too a,s an element in flinty crushes. A series of hornblende schist belts occur in the Laxdale Stream, varying in width from 40 feet and less. They penetrate the metamorphic series and run concurrently for long distances in à north -north -east direction. From their mineral contents they are reminiscent of belts in llorth and South Uist, east of the flinty zone. Peridotites occur at several localities; amongst gneisses in the Park district; at Thamanab- -haidh; and near Stornoway. Anorthosite masses are exposed at Skigersta and farther south on the coast line; Eye Peninsula; Park district, though on a lesser scale than on the Uist Hills, and Roineval, Harris. Some of these anorthosite rocks are involved in the flinty crush. Basaltic dykes have an important place in rock types of Lewis. Varying in character, they are distributed in great numbers, radiating in north -west lines - though not so prominently as in the southern part/ (8). part of the Long Island. Two of the best known pass through Broad Bay. At Loch Shell they are numerous, where at one part on the north shore twenty may be noted in the course of a mile. ivlicroscopically many of the basic masses are of exceptional interest. Some show an associa- tion of biotite, olivine, spinel, enstatite, and augite; others olivine and enstatite; olivine, biotite, pyroxene and felspar; olivine and hornblende, or pure olivine. Apparently they represent the counterpart of thé ultrabasic rocks described in the North -west Highland Geological Survey Memoir. (9) Analyses of Basic Igneous rock types. A. B. C. S i O2 43.40 39.04 39.3 A1203 6.82 3.00 4.84 Fe203 6.41 )12.80 )10.96 Fe0 8.26 23.55 27.40 32.00 0a0 4.33 6.32 2.90 TVa20 1.16 n.d. n.d. K20 1.25 n.d. n.d. Mn0 2.42 0.56 tr. Cû203 0.22 - - TiO2 0.43 - - P205 0.08 0.05 S 0.22 0.10 0.10 V205 0.18 - NiO 0.59 0.30 0.40 traces metallic oxides. 0.20 volatile matterl 0.50 10.47 7.80 ti20 100.02 100.04 98.30 Less Oxygen 0.06 0.02 0.02 9 .96 101.02 98.28 A. Mïaaruig ultra basic mass. B. Tob Lieravay - metamorphosed- actinolitic and chloritised bands, (anal. 1906). 0. Quarry 4 miles west of Stornoway, (anal. 1906). (lo). Analyses ofiasic Igneous i'Xpes A. B. C . SiO 50.32 2 48.80 42.18 A1203 11.68 26.00 2.36 Fe203 1.21 4.00 5.96 FeO 10.08 Ni1. 5.6 Mg0 19.87 0.66 34.81 Ca0 5.11 14.66 2.14 Na20 tr. 0.08 0.94 K20 tr. 3.68 0.26 1120 0.40 2.30 5.65 Mn0 0.37 - 0.18 `l' i02 0.27 - tr. 0r203 0.60 - tr. id i 0 0.20 - P206 - tr. - 100.11 100.18 100.08 A. Gabbro rock - Breascleit, Lewis. B. Anorthositic rock - Geall, Lewis. C. Peridotite - Thamanabhaidh, Lewis. Analyses of Basic bock Types. A. B. C. i(2 50.03 48.64 58.20 A1203 14.15 13.39 13.98 i+'8203 4.00 1.97 ) 10.72 Fe0 6.43 10.14 Mg0 7.29 10.24 5.00 °a0 10.58 9.71 5.90 volatile at 10°C0 .) 1.40 2.04 6.00 X120 etc. Na20 2.74 3.24 ) n.d.